IONS AND SALT ACTION. 75 



with alkaline reaction. Complete investigations on this subject have 

 been carried out by SORENSEN. 1 He found the value io~ 14>14 for K 

 at 18 C. CH is determined in either of two ways. The best method, 

 the electromotive, is based upon the electromotive force of gas chains, 

 as developed by NERNST.; 2 namely, if platinum foil covered with 

 platinum black is introduced into a watery solution and this saturated 

 with hydrogen, then a difference of electrical potential is produced between 

 the platinum and the solution and this potential is theoretically propor- 

 tional to the concentration of the hydrogen ions in the solution. We 

 cannot give any further detail as to this theory or to the performance 

 of the measurement of the difference in potential. 3 If the concentration 

 of the hydrogen ions CH is expressed in gram ions per liter by the figure 

 10~ p , then according to the suggestion of SORENSON the name hydrogen 

 ion exponent and the symbol pn is used for the numerical value of the 

 exponents of this potence. The relationship between p H and the electro- 

 motive force TT at the contact between the platinum and the solution 

 can be expressed graphically by a straight line; hence it follows that if 

 TT is known then p H can be very easily found (the exponential line). 



The other method used by SORENSEN 4 for the determination of CH 

 is a colorimetric method and depends on the use of indicators. After 

 much investigation 20 indicators are recommended, of which certain ones 

 require strictly fixed methods of use. As soon as more than a qual- 

 itative approximation is required then the shade of color produced by 

 the indicator must be compared with a shade of color produced by the 

 same indicator in a solution of known concentration of H ions. Such 

 standard solutions which allow of a variation in the concentration of the 

 H ions at one's pleasure have been given by SORENSEN, and the original 

 article gives a table of curve's from the corresponding value for p& which 

 can be read off, when the composition of a standard solution is known. The 

 figure PH for the standard solutions is determined by aid of the electro- 

 motive method. Standard solutions are selected so that they serve as 

 natural protectors against too sudden changes in pa (so called buffer) 5 . 



As above stated the dissociation constant according to SORENSEN 

 for water is 10~ 1414 at 18 C. or C H - C OH = 10~ 14 - 14 . In neutral reac- 

 tion C H = COH and therefore C H = 10~ 7 ' 07 or pn = 7.Q7. Smaller values 

 for pn correspond to acid and greater values to alkaline reaction. 



HASSELBACH 6 has suggested a modification of SORENSEN'S method 



1 Bioch. Zeitschr., 21, 131 (1909) also Ergebn. d. Physiol. Vol. 11. 



2 Zeitschr. f. physik. Chem., 4, 129 (1889). 



3 In regard to the determination see the work of Sorensen cited on page 74. 



4 Sorensen, Enzymstudien, Bioch. Zeitschr., 21, 253. 

 * Ibid., 167. 



6 Bioch. Zeitschr., 30, 317 (1910). 



